This invention relates to a process for the production of hydroxydiphenyl.
Hydroxydiphenyl is known to be obtained by catalytically dehydrogenating compounds or mixtures of compounds comprising completely and/or partially hydrogenated hydroxydiphenyl, in the gaseous phase. The dehydrogenation catalysts used for this purpose contain nickel, chromium, aluminium, copper and alkali oxide or carbonate and, in some cases, silver and are described, for example, in German Patent No. 1,108,221 and DOS No. 2,049,809.
In addition to hydroxydiphenyl, a number of secondary products and intermediate products, principally phenol, diphenyl and cyclohexylphenol and, to some extent, diphenylene oxide, are formed in this dehydrogenation process and have to be removed by special purification processes, for example the process described in DOS No. 2,102,476. However, the reaction product to be purified must satisfy certain qualitative requirements if a purification process of this kind is to be carried out successfully both from the technical and the economic standpoint. The main requirement is that the reaction product to be purified should have as uniform a qualitative and quantitative composition as possible.
On the other hand, the activity of the dehydrogenation catalyst is known to decrease with time so that the conversion is reduced and the composition of the reaction product is altered accordingly. Although this change in composition can be counteracted by reducing the catalyst load and/or by increasing the reaction temperature, reduction of the catalyst load involves a reduction in throughput and, hence, has an adverse effect upon the dehydrogenation process, whilst increasing the reaction temperature promotes secondary reactions to a greater extent than the main reaction, resulting in a deterioration in the composition of the reaction product.
For this reason, the continuous process has to be interrupted after a certain time in order to regenerate or replace the catalyst. However, this again involves a reduction in the output of the installation in which the process is carried out, quite apart from increased costs.
It has also proved to be advantageous to use a crude dehydrogenation product of substantially constant composition for purification. As already mentioned, however, it is not possible to obtain a crude product of this kind by conventional dehydrogenation processes. Although it is possible, by storing and mixing dehydrogenation products of different composition in an intermediate stage, to prepare a product of constant composition for the purification process, this involves more work and additional costs.
In order to obviate the disadvantages attending conventional dehydrogenation processes, there is an urgent need for a dehydrogenation process which gives a dehydrogenation product remaining uniform in its composition over prolonged periods without any appreciable change in the activity and selectivity of the dehydrogenation catalyst, which, in addition, should have a long service life.